Steel for hot stamping with enhanced oxidation resistance

ABSTRACT

An alloy composition is provided. The alloy composition includes chromium (Cr) at a concentration of greater than or equal to about 0.5 wt. % to less than or equal to about 9 wt. %, carbon (C) at a concentration of greater than or equal to about 0.15 wt. % to less than or equal to about 0.5 wt. %, manganese (Mn) at a concentration of greater than or equal to about 0 wt. % to less than or equal to about 3 wt. %, silicon (Si) at a concentration of greater than or equal to about 0.5 wt. % to less than or equal to about 2 wt. %, and a balance of the alloy composition being iron. Methods of making shaped steel objects from the alloy composition are also provided.

INTRODUCTION

This section provides background information related to the presentdisclosure which is not necessarily prior art.

Press-hardening steel (PHS), also referred to as “hot-stamped steel” or“hot formed steel,” is one of the strongest steels used for automotivebody structural applications, having tensile strength properties on theorder of about 1,500 mega-Pascal (MPa). Such steel has desirableproperties, including forming steel components with significantincreases in strength-to-weight ratios. PHS components have become evermore prevalent in various industries and applications, including generalmanufacturing, construction equipment, automotive or othertransportation industries, home or industrial structures, and the like.For example, when manufacturing vehicles, especially automobiles,continual improvement in fuel efficiency and performance is desirable,thus PHS components have been increasingly used. PHS components areoften used for forming load-bearing components, like door beams, whichusually require high strength materials. Thus, the finished state ofthese steels are designed to have high strength and enough ductility toresist external forces, for example, to resist intrusion into thepassenger compartment without fracturing so as to provide protection tothe occupants. Moreover, galvanized PHS components may provide cathodicprotection.

Many PHS processes involve austenitization in a furnace of a sheet steelblank immediately followed by pressing and quenching of the sheet indies. There are two main types of PHS processes: indirect and direct.Austenitization is typically conducted in the range of about 880° C. to950° C. Under the direct method, the PHS component is formed and pressedsimultaneously between dies, which quenches the steel. Under theindirect method, the PHS component is cold formed to an intermediatepartial shape before austenitization and the subsequent pressing andquenching steps. The quenching of the PHS component hardens thecomponent by transforming the microstructure from austenite tomartensite. An oxide layer often forms during the transfer from thefurnace to the dies. After quenching, therefore, the oxide must beremoved from the PHS component and the dies. The oxide is typicallyremoved, i.e., descaled, by shot blasting.

The PHS component may be made from bare or aluminum-silicon (Al—Si)alloy using the direct method or from zinc (Zn) coated PHS using thedirect method or an indirect method. Coating the PHS component providesa protective layer (e.g., galvanic protection) to the underlying steelcomponent. Zinc coatings offer cathodic protection; the coating acts asa sacrificial layer and corrodes instead of the steel component, evenwhere the steel is exposed. Such coatings also generate oxides on PHScomponents' surfaces, which are removed by shot blasting. Accordingly,alloy compositions that do not require coatings or other treatments aredesired.

SUMMARY

This section provides a general summary of the disclosure, and is not acomprehensive disclosure of its full scope or all of its features.

In various aspects, the present technology provides an alloy compositionincluding chromium (Cr) at a concentration of greater than or equal toabout 0.5 wt. % to less than or equal to about 9 wt. %; carbon (C) at aconcentration of greater than or equal to about 0.15 wt. % to less thanor equal to about 0.5 wt. %; manganese (Mn) at a concentration ofgreater than or equal to about 0 wt. % to less than or equal to about 3wt. %; silicon (Si) at a concentration of greater than or equal to about0.5 wt. % to less than or equal to about 2 wt. %; and a balance of thealloy composition being iron.

In one aspect, the alloy composition includes Si at a concentration ofgreater than or equal to about 0.6 wt. % to less than or equal to about1.5 wt. %.

In one aspect, the alloy composition includes Cr at a concentration ofgreater than or equal to about 2 wt. % to less than or equal to about 3wt. %.

In one aspect, the alloy composition further includes aluminum (Al) at aconcentration of greater than or equal to about 0 wt. % to less than orequal to about 5 wt. %.

In one aspect, the alloy composition further includes nitrogen (N) at aconcentration of greater than or equal to about 0 wt. % to less than orequal to about 0.01 wt. %.

In one aspect, the alloy composition further includes at least one of:molybdenum (Mo) at a concentration of greater than or equal to about 0wt. % to less than or equal to about 1 wt. %; nickel (Ni) at aconcentration of greater than or equal to about 0 wt. % to less than orequal to about 1 wt. %; boron (B) at a concentration of greater than orequal to about 0 wt. % to less than or equal to about 0.01 wt. %;niobium (Nb) at a concentration of greater than or equal to about 0 wt.% to less than or equal to about 0.5 wt. %; and vanadium (V) at aconcentration of greater than or equal to about 0 wt. % to less than orequal to about 0.5 wt. %.

In one aspect, the alloy composition is in the form of an alloy coil.

In one aspect, the alloy coil includes ferrite, martensite and retainedaustenite (RA).

In one aspect, the alloy composition has been subjected to a quench andpartitioning process.

In one aspect, a hot stamping method of forming a shaped steel object isprovided. The hot stamping method includes austenitizing a blank havingthe alloy composition, stamping the austenitized blank to form a shapedobject, and quenching the shaped object to form the shaped steel object.

In one aspect, a cold stamping method of forming a shaped steel objectis provided. The cold stamping method includes cutting a blank from acoil having the alloy composition, wherein the alloy composition hasbeen subjected to a quench and partitioning process; and stamping theblank into a predetermined shape at ambient temperature to form theshaped steel object.

In various aspects, the present technology also provides a method offorming a shaped steel object; the method including cutting a blank froma coil including an alloy composition having chromium (Cr) at aconcentration of greater than or equal to about 0.5 wt. % to less thanor equal to about 9 wt. %, carbon (C) at a concentration of greater thanor equal to about 0.15 wt. % to less than or equal to about 0.5 wt. %,manganese (Mn) at a concentration of greater than or equal to about 0wt. % to less than or equal to about 3 wt. %, silicon (Si) at aconcentration of greater than or equal to about 0.5 wt. % to less thanor equal to about 2 wt. %, and a balance of the alloy composition beingiron; heating the blank to a temperature above an upper criticaltemperature (Ac3) of the alloy composition to form a heated blank havingaustenite; stamping the heated blank into a predetermined shape to forma stamped object; and quenching the stamped object to form the shapedsteel object, wherein the shaped steel object includes martensite.

In one aspect, the quenching includes decreasing the temperature of thestamped object at a rate of greater than or equal to about 15° C./suntil the stamped object reaches a temperature below a martensite finish(Mf) temperature of the alloy composition.

In one aspect, the method is free from pre-oxidizing the alloycomposition, coating the shaped steel object, and shot blasting.

In one aspect, the quenching has a quench and partitioning process,wherein the quench and partitioning process includes decreasing thetemperature of the stamped object until the stamped object has atemperature between a martensite start (Ms) temperature of the alloycomposition and a martensite finish (Mf) temperature of the alloycomposition; incubating the stamped object at a partitioning temperaturewherein carbon (C) is partitioned from martensite into austenite; anddecreasing an austenite Mf temperature to a temperature below roomtemperature.

In one aspect, the quench and partitioning process forms the shapedsteel object, wherein the shaped steel object includes ferrite,martensite and retained austenite (RA).

In one aspect, the shaped steel object is substantially free ofcementite.

In various aspects, the present technology yet further provides a methodof forming a shaped steel object; the method including cutting a blankfrom a coil of an advanced high strength steel (AHSS); and stamping theblank into a predetermined shape at ambient temperature to form theshaped steel object, wherein the AHSS is made by subjecting an alloycomposition to a quench and partitioning process, the alloy compositionhaving chromium (Cr) at a concentration of greater than or equal toabout 0.5 wt. % to less than or equal to about 9 wt. %, carbon (C) at aconcentration of greater than or equal to about 0.15 wt. % to less thanor equal to about 0.5 wt. %, manganese (Mn) at a concentration ofgreater than or equal to about 0 wt. % to less than or equal to about 3wt. %, silicon (Si) at a concentration of greater than or equal to about0.5 wt. % to less than or equal to about 2 wt. %, and a balance of thealloy composition being iron.

In one aspect, the AHSS is substantially free of an oxide layer.

In one aspect, the shaped steel object is bare of zinc (Zn) coated.

Further areas of applicability will become apparent from the descriptionprovided herein. The description and specific examples in this summaryare intended for purposes of illustration only and are not intended tolimit the scope of the present disclosure.

DRAWINGS

The drawings described herein are for illustrative purposes only ofselected embodiments and not all possible implementations, and are notintended to limit the scope of the present disclosure.

FIG. 1 is a graph showing a temperature versus time for a traditionalhot stamping method and a hot stamping method including quench andpartitioning.

FIG. 2A is an image of a steel made from a high chromium content, lowsilicon content alloy without pre-oxidation.

FIG. 2B is an image of a steel made from a high chromium content, lowsilicon content alloy with pre-oxidation.

FIG. 3A is an image of a steel made from an alloy composition accordingto various aspects of the current technology (2% Cr, 0.6% Si) withoutpre-oxidation.

FIG. 3B is an image of a steel made from an alloy composition accordingto various aspects of the current technology (3% Cr, 0.6% Si) withoutpre-oxidation.

FIG. 3C is an image of a steel made from an alloy composition accordingto various aspects of the current technology (3% Cr, 1.5% Si) withoutpre-oxidation.

FIG. 4 shows cross sectional images of a steel made from an alloycomposition according to various aspects of the current technology (2%Cr, 0.6% Si) without pre-oxidation.

FIG. 5A is a cross-sectional image of a steel made from an alloycomposition according to various aspects of the current technology (2%Cr, 0.6% Si) without pre-oxidation.

FIG. 5B is a cross-sectional image of a steel made from an alloycomposition according to various aspects of the current technology (3.1%Cr, 0.61% Si) without pre-oxidation.

FIG. 5C is a cross sectional image of a steel made from an alloycomposition according to various aspects of the current technology (3.2%Cr, 1.46% Si) without pre-oxidation.

FIG. 6A is a graph showing thermodynamics of an alloy system that doesnot comprise silicon.

FIG. 6B is a graph showing thermodynamics of an alloy system thatcomprises silicon according to various aspects of the currenttechnology.

FIG. 7 shows aspects of a method for making a shaped steel objectaccording to various aspects of the current technology.

Corresponding reference numerals indicate corresponding parts throughoutthe several views of the drawings.

DETAILED DESCRIPTION

Example embodiments are provided so that this disclosure will bethorough, and will fully convey the scope to those who are skilled inthe art. Numerous specific details are set forth such as examples ofspecific compositions, components, devices, and methods, to provide athorough understanding of embodiments of the present disclosure. It willbe apparent to those skilled in the art that specific details need notbe employed, that example embodiments may be embodied in many differentforms and that neither should be construed to limit the scope of thedisclosure. In some example embodiments, well-known processes,well-known device structures, and well-known technologies are notdescribed in detail.

The terminology used herein is for the purpose of describing particularexample embodiments only and is not intended to be limiting. As usedherein, the singular forms “a,” “an,” and “the” may be intended toinclude the plural forms as well, unless the context clearly indicatesotherwise. The terms “comprises,” “comprising,” “including,” and“having,” are inclusive and therefore specify the presence of statedfeatures, elements, compositions, steps, integers, operations, and/orcomponents, but do not preclude the presence or addition of one or moreother features, integers, steps, operations, elements, components,and/or groups thereof. Although the open-ended term “comprising,” is tobe understood as a non-restrictive term used to describe and claimvarious embodiments set forth herein, in certain aspects, the term mayalternatively be understood to instead be a more limiting andrestrictive term, such as “consisting of” or “consisting essentiallyof.” Thus, for any given embodiment reciting compositions, materials,components, elements, features, integers, operations, and/or processsteps, the present disclosure also specifically includes embodimentsconsisting of, or consisting essentially of, such recited compositions,materials, components, elements, features, integers, operations, and/orprocess steps. In the case of “consisting of,” the alternativeembodiment excludes any additional compositions, materials, components,elements, features, integers, operations, and/or process steps, while inthe case of “consisting essentially of” any additional compositions,materials, components, elements, features, integers, operations, and/orprocess steps that materially affect the basic and novel characteristicsare excluded from such an embodiment, but any compositions, materials,components, elements, features, integers, operations, and/or processsteps that do not materially affect the basic and novel characteristicscan be included in the embodiment.

Any method steps, processes, and operations described herein are not tobe construed as necessarily requiring their performance in theparticular order discussed or illustrated, unless specificallyidentified as an order of performance. It is also to be understood thatadditional or alternative steps may be employed, unless otherwiseindicated.

When a component, element, or layer is referred to as being “on,”“engaged to,” “connected to,” or “coupled to” another element or layer,it may be directly on, engaged, connected or coupled to the othercomponent, element, or layer, or intervening elements or layers may bepresent. In contrast, when an element is referred to as being “directlyon,” “directly engaged to,” “directly connected to,” or “directlycoupled to” another element or layer, there may be no interveningelements or layers present. Other words used to describe therelationship between elements should be interpreted in a like fashion(e.g., “between” versus “directly between,” “adjacent” versus “directlyadjacent,” etc.). As used herein, the term “and/or” includes any and allcombinations of one or more of the associated listed items.

Although the terms first, second, third, etc. may be used herein todescribe various steps, elements, components, regions, layers and/orsections, these steps, elements, components, regions, layers and/orsections should not be limited by these terms, unless otherwiseindicated. These terms may be only used to distinguish one step,element, component, region, layer or section from another step, element,component, region, layer or section. Terms such as “first,” “second,”and other numerical terms when used herein do not imply a sequence ororder unless clearly indicated by the context. Thus, a first step,element, component, region, layer or section discussed below could betermed a second step, element, component, region, layer or sectionwithout departing from the teachings of the example embodiments.

Spatially or temporally relative terms, such as “before,” “after,”“inner,” “outer,” “beneath,” “below,” “lower,” “above,” “upper,” and thelike, may be used herein for ease of description to describe one elementor feature's relationship to another element(s) or feature(s) asillustrated in the figures. Spatially or temporally relative terms maybe intended to encompass different orientations of the device or systemin use or operation in addition to the orientation depicted in thefigures.

Throughout this disclosure, the numerical values represent approximatemeasures or limits to ranges to encompass minor deviations from thegiven values and embodiments having about the value mentioned as well asthose having exactly the value mentioned. Other than in the workingexamples provided at the end of the detailed description, all numericalvalues of parameters (e.g., of quantities or conditions) in thisspecification, including the appended claims, are to be understood asbeing modified in all instances by the term “about” whether or not“about” actually appears before the numerical value. “About” indicatesthat the stated numerical value allows some slight imprecision (withsome approach to exactness in the value; approximately or reasonablyclose to the value; nearly). If the imprecision provided by “about” isnot otherwise understood in the art with this ordinary meaning, then“about” as used herein indicates at least variations that may arise fromordinary methods of measuring and using such parameters. For example,“about” may comprise a variation of less than or equal to 5%, optionallyless than or equal to 4%, optionally less than or equal to 3%,optionally less than or equal to 2%, optionally less than or equal to1%, optionally less than or equal to 0.5%, and in certain aspects,optionally less than or equal to 0.1%.

In addition, disclosure of ranges includes disclosure of all values andfurther divided ranges within the entire range, including endpoints andsub-ranges given for the ranges.

Example embodiments will now be described more fully with reference tothe accompanying drawings.

To overcome the necessity to coat PHS alloys, an alloy with a highchromium concentration is described. The high chromium concentrationalloy comprises chromium at a concentration of greater than or equal toabout 2 wt. % to less than or equal to about 10 wt. % of the alloycomposition, aluminum at a concentration of greater than or equal toabout 0 wt. % to less than or equal to about 5 wt. % of the alloycomposition, carbon at a concentration of greater than or equal to about0.15% by weight to less than or equal to about 0.5 wt. % of the alloycomposition, and a balance of the high chromium concentration alloybeing iron. Although the high chromium concentration alloy does notrequire coating or shot blasting, it does require pre-oxidation byincubating the high chromium concentration alloy at a temperature ofgreater than or equal to about 400° C. to less than or equal to about700° C. for a time of greater than or equal to about 1 minute to lessthan or equal to about 60 minutes.

Accordingly, the current technology relates to an alloy compositionhaving a high chromium content that is suitable for hot and coldstamping applications, that does not require coating or shot blastingfor hot stamping applications, and that is resistant to oxidation, i.e.,does not require pre-oxidation prior to being press hardened. The alloycomposition has a high chromium content to preclude a coatingrequirement, and also includes a high silicon (Si) content for improvingoxidation resistance. The high silicon content also permits the chromiumconcentration to be decreased.

In various aspects of the current technology, the alloy composition isin a form of a blank for hot stamping processes. Here, the blank forms apress hardening steel after hot stamping processes. Components withinthe alloy composition, such as, for example, boron and chromium, lower acritical cooling rate in hot stamping processes relative to criticalcooling rates employed without such components. In other aspects of thecurrent technology, the alloy composition is in a form of a blank forcold stamping processes. Here, the blank is an advance high strengthsteel (AHSS) for cold stamping.

The alloy composition of the current technology comprises silicon (Si)at a concentration of greater than or equal to about 0.5 wt. % to lessthan or equal to about 2 wt. %, greater than or equal to about 0.6 wt. %to less than or equal to about 1.8 wt. %, or greater than or equal toabout 0.8 wt. % to less than or equal to about 1.5 wt. %. For example,in various embodiments the alloy composition comprises Si at aconcentration of about 0.5 wt. %, about 0.6 wt. %, about 0.7 wt. %,about 0.8 wt. %, about 0.9 wt. %, about 1 wt. %, about 1.1 wt. %, about1.2 wt. %, about 1.3 wt. %, about 1.4 wt. %, about 1.5 wt. %, about 1.6wt. %, about 1.7 wt. %, about 1.8 wt. %, about 1.9 wt. %, or about 2 wt.%. This high amount of Si in the alloy composition improves oxidationresistance, permits a lower amount of chromium to be added while stillnot requiring coating or shot blasting after forming, and prevents,inhibits, or decreases cementite formation during a quench andpartitioning process.

The alloy composition also comprises chromium (Cr). Without the highlevels of Si, Cr would have to be added at a level of from about 2 wt. %to about 10 wt. % to prevent the need to coat and/or shot blast. Becauseof the high levels of Si; however, the alloy composition comprises Cr ata concentration of greater than or equal to about 0.5 wt. % to less thanor equal to about 9 wt. %, greater than or equal to about 1.5 wt. % toless than or equal to about 8 wt. %, greater than or equal to about 1.75wt. % to less than or equal to about 5 wt. %, greater than or equal toabout 2 wt. % to less than or equal to about 4 wt. %, or greater than orequal to about 2 wt. % to less than or equal to about 3 wt. %. Forexample, in various embodiments the alloy composition comprises Cr at aconcentration of about 0.5 wt. %, about 1 wt. %, about 1.5 wt. %, about2 wt. %, about 2.5 wt. %, about 3 wt. %, about 3.5 wt. %, about 4 wt. %,about 4.5 wt. %, about 5 wt. %, about 5.5 wt. %, about 6 wt. %, about6.5 wt. %, about 7 wt. %, about 7.5 wt. %, about 8 wt. %, about 8.5 wt.%, or about 9 wt. %.

The alloy composition also comprises carbon (C) at a concentration ofgreater than or equal to about 0.15 wt. % to less than or equal to about0.5 wt. %; greater than or equal to about 0.15 wt. % to less than orequal to about 0.45 wt. %, greater than or equal to about 0.15 wt. % toless than or equal to about 0.4 wt. %, greater than or equal to about0.15 wt. % to less than or equal to about 0.3 wt. %, greater than orequal to about 0.15 wt. % to less than or equal to about 0.25 wt. %, orgreater than or equal to about 0.15 wt. % to less than or equal to about0.2 wt. %. For example, in various embodiments the alloy compositioncomprises C at a concentration of about 0.15 wt. %, about 0.2 wt. %,about 0.25 wt. %, about 0.3 wt. %, about 0.35 wt. %, about 0.4 wt. %,about 0.45 wt. %, or about 0.5 wt. %.

The alloy composition can also include manganese (Mn) at a concentrationof greater than or equal to about 0 wt. % to less than or equal to about3 wt. %, greater than or equal to about 0.25 wt. % to less than or equalto about 2.5 wt. %, greater than or equal to about 0.5 wt. % to lessthan or equal to about 2 wt. %, greater than or equal to about 0.75 wt.% to less than or equal to about 1.5 wt. %, or greater than or equal toabout 1 wt. % to less than or equal to about 1.5 wt. %. In someembodiments, the alloy composition is substantially free of Mn. As usedherein, “substantially free” refers to trace component levels, such aslevels of less than or equal to about 1.5%, less than or equal to about1%, less than or equal to about 0.5%, or levels that are not detectable.In various embodiments, the alloy composition is substantially free ofMn or comprises Mn at a concentration of less than or equal to about 0.5wt. %, less than or equal to about 1 wt. %, less than or equal to about1.5 wt. %, less than or equal to about 2 wt. %, less than or equal toabout 2.5 wt. %, or less than or equal to about 3 wt. %.

In various embodiments, the alloy composition further comprises aluminum(Al) at a concentration of greater than or equal to about 0 wt. % toless than or equal to about 5 wt. %, from greater than or equal to about0.1 wt. % to less than or equal to about 4.5 wt. %, from greater than orequal to about 1 wt. % to less than or equal to about 4 wt. %, fromgreater than or equal to about 2 wt. % to less than or equal to about 3wt. %, from greater than or equal to about 0 wt. % to less than or equalto about 0.1 wt. %, from greater than or equal to about 0.015 wt. % toless than or equal to about 0.075 wt. %, or from greater than or equalto about 0.02 wt. % to less than or equal to about 0.05 wt. %. Forexample, in various embodiments the alloy composition is substantiallyfree of Al or comprises Al at a concentration of about less than orequal to 0.5 wt. %, less than or equal to about 1 wt. %, less than orequal to about 1.5 wt. %, less than or equal to about 2 wt. %, less thanor equal to about 2.5 wt. %, less than or equal to about 3 wt. %, lessthan or equal to about 3.5 wt. %, less than or equal to about 4 wt. %,less than or equal to about 4.5 wt. %, or less than or equal to about 5wt. %.

In various embodiments, the alloy composition further comprises nitrogen(N) at a concentration of greater than or equal to about 0 wt. % to lessthan or equal to about 0.01 wt. %. For example, in various embodimentsthe alloy composition is substantially free of N or comprises N at aconcentration of less than or equal to about 0.001 wt. %, less than orequal to 0.002 wt. %, less than or equal to 0.003 wt. %, less than orequal to 0.004 wt. %, less than or equal to 0.005 wt. %, less than orequal to 0.006 wt. %, less than or equal to 0.007 wt. %, less than orequal to 0.008 wt. %, less than or equal to 0.009 wt. %, or less than orequal to 0.01 wt. %.

In various embodiments, the alloy composition further comprisesmolybdenum (Mo) at a concentration of greater than or equal to about 0wt. % to less than or equal to about 1 wt. %, or less than or equal toabout 0.8 wt. %. For example, in various embodiments the alloycomposition is substantially free of Mo or comprises Mo at aconcentration of less than or equal to about 0.1 wt. %, less than orequal to about 0.2 wt. %, less than or equal to about 0.3 wt. %, lessthan or equal to about 0.4 wt. %, less than or equal to about 0.5 wt. %,less than or equal to about 0.6 wt. %, less than or equal to about 0.7wt. %, less than or equal to about 0.8 wt. %, less than or equal toabout 0.9 wt. %, or less than or equal to about 1.0 wt. %.

In various embodiments, the alloy composition further comprises nickel(Ni) at a concentration of greater than or equal to about 0 wt. % toless than or equal to about 1 wt. %, or less than or equal to about 0.8wt. %. For example, in various embodiments the alloy composition issubstantially free of Ni or comprises Ni at a concentration of less thanor equal to about 0.1 wt. %, less than or equal to about 0.2 wt. %, lessthan or equal to about 0.3 wt. %, less than or equal to about 0.4 wt. %,less than or equal to about 0.5 wt. %, less than or equal to about 0.6wt. %, less than or equal to about 0.7 wt. %, less than or equal toabout 0.8 wt. %, less than or equal to about 0.9 wt. %, or less than orequal to about 1.0 wt. %.

In various embodiments, the alloy composition further comprises boron(B) at a concentration of greater than or equal to about 0 wt. % to lessthan or equal to about 0.01 wt. %, or less than or equal to about 0.005wt. %. For example, in various embodiments the alloy composition issubstantially free of B or comprises B at a concentration of less thanor equal to about 0.001 wt. %, less than or equal to about 0.002 wt. %,less than or equal to about 0.003 wt. %, less than or equal to about0.004 wt. %, less than or equal to about 0.005 wt. %, less than or equalto about 0.006 wt. %, less than or equal to about 0.007 wt. %, less thanor equal to about 0.008 wt. %, less than or equal to about 0.009 wt. %,or less than or equal to about 0.01 wt. %.

In various embodiments, the alloy composition further comprises niobium(Nb) at a concentration of greater than or equal to about 0 wt. % toless than or equal to about 0.5 wt. %, or less than or equal to about0.3 wt. %. For example, in various embodiments the alloy composition issubstantially free of Nb or comprises Nb at a concentration of less thanor equal to about 0.1 wt. %, less than or equal to about 0.2 wt. %, lessthan or equal to about 0.3 wt. %, less than or equal to about 0.4 wt. %,or less than or equal to about 0.5 wt. %.

In various embodiments, the alloy composition further comprises vanadium(V) at a concentration of greater than or equal to about 0 wt. % to lessthan or equal to about 0.5 wt. %, or less than or equal to about 0.3 wt.%. For example, in various embodiments the alloy composition issubstantially free of V or comprises V at a concentration of less thanor equal to about 0.1 wt. %, less than or equal to about 0.2 wt. %, lessthan or equal to about 0.3 wt. %, less than or equal to about 0.4 wt. %,or less than or equal to about 0.5 wt. %.

In various embodiments, the alloy composition comprises at least one ofMn, Al, N, Mo, Ni, B, Nb, and V, or at least one of Mo, Ni, B, Nb, andV.

A balance of the alloy composition is iron.

Table 1 shows the composition of the alloy composition relative to abaseline high chromium press hardened steel (PHS).

TABLE 1 Composition of baseline high chromium PHS and an alloycomposition according to the present technology. Chemical Composition(wt. %) Grade Coating C Mn Cr Si N Others Baseline Cr PHS Free 0.1-0.450-3.0   2-10 0-0.5 <0.006 Cr PHS Free 0.1-0.45 0-3.0 0.5-9 0.5-2   <0.006 Mo < 0.8, (current technology) B < 0.005, Nb/V < 0.3

The alloy composition can include various combinations of Si, Cr, C, Mn,Al, N, Mo, Ni, B, Nb, V, and Fe at their respective concentrationsdescribed above. In some embodiments, the alloy composition consistsessentially of Si, Cr, C, Mn, and Fe. As described above, the term“consists essentially of” means the alloy composition precludesadditional compositions, materials, components, elements, and/orfeatures, that materially affect the basic and novel characteristics ofthe alloy composition, such as the alloy composition not requiringpre-oxidation, coating, or shot blasting when formed into a shapedobject, but any compositions, materials, components, elements, and/orfeatures, that do not materially affect the basic and novelcharacteristics can be included in the embodiment. Therefore, when thealloy composition consists essentially of Si, Cr, C, Mn, and Fe, thealloy composition can also include any combination of Al, N, Mo, Ni, B,Nb, and V that does not materially affect the basic and novelcharacteristics of the alloy composition. In other embodiments, thealloy composition consists of Si, Cr, C, Mn, Fe in their respectiveconcentrations described above and at least one of Al, N, Mo, Ni, B, Nb,and V in no more than trace amounts, such as levels of less than orequal to about 1.5%, less than or equal to about 1%, less than or equalto about 0.5%, or levels that are not detectable. Other elements thatare not described herein can also be included in trace amounts with theproviso that they do not materially affect the basic and novelcharacteristics of the alloy composition.

In one embodiment, the alloy composition consists essentially of Si, Cr,C, Mn, and Fe. In another embodiment, the alloy composition consists ofSi, Cr, C, Mn, and Fe.

In one embodiment, the alloy composition consists essentially of Si, Cr,C, Mn, Al, and Fe. In another embodiment, the alloy composition consistsof Si, Cr, C, Mn, Al and Fe.

In one embodiment, the alloy composition consists essentially of Si, Cr,C, Mn, Al, Mo, and Fe. In another embodiment, the alloy compositionconsists of Si, Cr, C, Mn, Al, Mo, and Fe

In one embodiment, the alloy composition consists essentially of Si, Cr,C, Mn, Al, Mo, Nb, V, and Fe. In another embodiment, the alloycomposition consists of Si, Cr, C, Mn, Al, Mo, Nb, V, and Fe.

In one embodiment, the alloy composition consists essentially of Si, Cr,C, Mn, Al, Mo, Ni, Nb, V, and Fe. In another embodiment, the alloycomposition consists of Si, Cr, C, Mn, Al, Mo, Ni, Nb, V, and Fe.

In one embodiment, the alloy composition consists essentially of Si, Cr,C, Mn, N, Ni, and Fe. In another embodiment, the alloy compositionconsists of Si, Cr, C, Mn, N, Ni, and Fe.

In one embodiment, the alloy composition consists essentially of Si, Cr,C, Mn, Al, N, Mo, Ni, B, Nb, V, and Fe. In another embodiment, the alloycomposition consists of Si, Cr, C, Mn, Al, N, Mo, Ni, B, Nb, V, and Fe.

In one embodiment, the alloy composition consists essentially of Si, Cr,C, and Fe. In another embodiment, the alloy composition consists of Si,Cr, C, and Fe.

In one embodiment, the alloy composition consists essentially of Si, Cr,C, Mo, B, Nb, V, and Fe. In another embodiment, the alloy compositionconsists of Si, Cr, C, Mo, B, Nb, V, and Fe.

In various aspects of the current technology, the alloy composition isin the form of a coil of the metal. In this form, the coil can beunrolled and cut into predetermined shapes or blanks. The blanks can behot stamped using a traditional quenching method or by a quench andpartitioning method. FIG. 1 shows a graph 10 having a y-axis 12representing temperature and an x-axis 14 representing time. A firstline 16 on the graph 10 represents a traditional process. Here, theblank is austenitized, i.e., heated to a final temperature 18 that isabove an upper critical temperature (Ac3) 20 of the alloy composition.The final temperature 18 is greater than or equal to about 880° C. toless than or equal to about 950° C. The austentized blank is thenstamped or hot formed into a shaped object at a temperature 22 betweenthe final temperature 18 and Ac3 20 and then cooled at a rate of greaterthan or equal to about 1° C.s⁻¹, greater than or equal to about 5° C.s⁻,greater than or equal to about 10° C.s⁻¹, greater than or equal to about15° C.s⁻¹, or greater than or equal to about 20° C.s⁻¹, such as at arate of about 1° C.s⁻¹, about 3° C.s⁻¹, about 5° C.s⁻¹, about 10° C.s⁻¹,about 15° C.s⁻¹, about, 20° C.s⁻¹ about, about 25° C.s⁻¹, about 30°C.s⁻¹ or faster until the temperature decreases below a martensite start(Ms) temperature 24, and below a martensite finish (Mf) temperature 26,such that the shaped object comprises a fully or substantially fullymartensite microstructure.

The graph 10 also includes a second line 28 representing a quench andpartition process. Here, the blank is austenitized at the finaltemperature 18, which is above the Ac3 temperature 20 of the alloycomposition. The austentized blank is then stamped or hot formed into ashaped object at a temperature 22 between the final temperature 18 andAc3 20 and then cooled at the rate described above for the traditionalprocess. However, when the temperature is decreased to a temperaturebetween the Ms temperature 24 and the Mf temperature 26, i.e., aftermartensite begins to form, but before the structure is fully martensite,the temperature is held constant, increased, or decreased slowly, suchthat a partitioning temperature is obtained in which carbon (C) ispartitioned from martensite into austenite. The temperature is thendecreased to a temperature below the Mf temperature 26. The resultingshaped object has a microstructure comprising martensite and retainedaustenite (RA) and a surface comprising a thin layer of oxide ofchromium (Cr) and silicon (Si). This oxide layer has a thickness of lessthan or equal to about 30 less than or equal to about 25 less than orequal to about 20 less than or equal to about 15 μm, less than or equalto about 10 μm, less than or equal to about 5 μm, or less than or equalto about 1 μm. The high silicon concentration in the alloy compositionprevents, inhibits, or decreases the formation of cementite in the finalmicrostructure when the quench and partitioning process is used. Neitherthe traditional process nor the quench and partitioning process requiresa pre-oxidation step or descaling step (such as by shot blasting).

In various aspects of the current technology, the alloy composition isaustentized and subjected to a quench and partitioning process to forman advanced high strength steel (AHSS), and then formed into a coil ofthe metal material. Here, the AHSS coil comprises ferrite, martensiteand retained austenite (RA) and is substantially free of an oxide layer.Being “substantially free” of an oxide layer means that the AHSScomprises an oxide layer with a thickness of less than or equal to about5 μm, less than or equal to about 2.5 μm, or less than or equal to about1 μm. This AHSS is suitable for making shaped objects by cold stampingat ambient temperature. The shaped objects can be bare or zinc (Zn)coated.

FIG. 2A is an image of a control alloy (3% Cr and 0.3% Si) that washeated at 900° C. for 10 minutes and then quenched traditionally. Here,the control alloy is not pre-oxidized. Rather, the high Cr alloycomposition is heated to 900° C. for 10 minutes and transferred to wateror oil cooled die for press forming and quenching. FIG. 2B shows asecond micrograph of a surface of a press hardening steel made from thecontrol alloy. Here, the control alloy is pre-oxidized at 500° C. for 20minutes, cooled, and then press hardened at 900° C. for 10 minutes, andthen cooled. As can be determined from the micrographs, the controlalloy comprising 0.3% Si requires peroxidation in order to achieve ahigh surface quality.

FIG. 3A, FIG. 3B, and FIG. 3C show alloy compositions comprising 2% Crand 0.6% Si, 3% Cr and 0.6% Si, and 3% Cr and 1.5% Si respectively.These are alloy compositions according to the present technology thatare not pre-oxidized and that are heated to 900° C. for 4-10 minutes andthen cooled. The surface quality is good for each of the alloycompositions with the quality increasing from FIG. 3A to FIG. 3B to FIG.3C. FIGS. 2A-2B and FIGS. 3A-3C show that the control alloy comprisingonly 0.3% Si requires pre-oxidation and the alloy composition of thecurrent technology does not require pre-oxidation.

FIG. 4 shows cross-section images of the quenched alloy composition inFIG. 3A. A first image 30 shows a thin surface layer on the quenchedalloy composition. A second image 32 shows an iron (Fe) distribution inthe surface layer. A third image 34 shows an oxygen (O) distribution inthe surface layer. A fourth image 36 shows a silicon (Si) distributionin the surface layer. A fifth image 38 shows a chromium (Cr)distribution in the surface layer. A high segregation of O, Si, and Crin these images 30, 32, 34, 36, 38 show that the surface layer comprisesa dense oxide of Cr and Si.

FIGS. 5A, 5B, and 5C show additional alloy compositions according to thepresent technology. The alloy compositions comprises 2% Cr and 0.6% Si,3.1% Cr and 0.61% Si, and 3.2% Cr and 1.46% Si in FIGS. 5A, 5B, and 5C,respectively, which are not pre-oxidized and that are heated to 900° C.for 10 minutes and then cooled. Each of the alloy compositions resultsin a high surface quality. Notably, whereas the alloy compositions ofFIGS. 5A and 5B generate thin oxide layers of about 20 μm thick, thealloy composition of FIG. 5C has an oxide layer of less than about 1 μmthick.

Without being bound by theory, adding high levels of Cr to the alloycomposition, such as, for example, about 3% Cr by weight of thecomposition decreases the austenitization temperature. FIG. 6A shows athermodynamics graph 40, wherein the x-axis 42 represents Crconcentration (from 0-12 wt. %) for a 0.22% C-1.5% Mn-xCr steel (withoutSi) and the y-axis 44 represents temperature (from 500-1000° C.). Afirst region 46 is shown for body-centered cubic (bcc)+face-centeredcubic (fcc) 0.22% C-1.5% Mn-xCr steel, a second region 48 is shown forbcc+M₇C₃ (carbide) steel, a third region 50 is shown for bcc+fcc+M₇C₃(carbide) steel, a fourth region 52 is shown for fcc+M₇C₃ (carbide)steel, and a fifth region 54 is shown for bcc+M₂₃C₆ (carbide) steel. Ahot stamping area 56 is shown for 0.22% C-1.5% Mn-xCr. According to thegraph, including Cr at a concentration of about 3% by weight of thealloy composition lowers the temperature required for hot stamping fromthe baseline temperature of about 800° C. to a fcc point of about 780°C. FIG. 6B shows a thermodynamics graph 60, wherein the x-axis 62represents Cr concentration (from 0-12 wt. %) for a 0.22% C-1.5% Mn-1.6%Si-xCr steel and the y-axis 64 represents temperature (from 500-1000°C.). A first region 66 is shown for bcc+fcc 0.22% C-1.5% Mn-xCr steel, asecond region 68 is shown for bcc+M₇C₃ (carbide) steel, a third region70 is shown for bcc+fcc+M₇C₃ (carbide) steel, and a fourth region 72 isshown for bcc+M₂₃C₆ (carbide) steel. A hot stamping area 74 is shown for0.22% C-1.5% Mn-1.6% Si-xCr. The graphs 40, 60 show that adding Si has aminimal effect on the Ac3 temperature of the alloy.

Hardened steel made from the alloy composition has an ultimate tensilestrength (UTS) of greater than or equal to about 1200 MPa, such as a UTSof about 1200 MPa, about 1250 MPa, about 1300 MPa, about 1350 MPa, about1400 MPa, about 1450 MPa, about 1500 MPa, about 1550 MPa, about 1600MPa, about 1650 MPa, about 1700 MPa, about 1750 MPa, about 1800 MPa,about 1850 MPa, about 1900 MPa, about 1950 MPa, about 2000 MPa, orgreater. Also, the hardened steel made from the alloy composition has aductility (elongation) of greater than or equal to about 4% (elongation)to less than or equal to about 10% (elongation), such as a ductility ofabout 4% (elongation), about 5% (elongation), about 6% (elongation),about 7% (elongation), about 8% (elongation), about 9% (elongation), orabout 10% (elongation) in the hardened condition.

With reference to FIG. 7, the current technology also provides a method80 of forming a shaped steel object. The shaped steel object can be anyobject that is generally made by hot stamping, such as, for example, avehicle part. Non-limiting examples of vehicles that have parts suitableto be produced by the current method include bicycles, automobiles,motorcycles, boats, tractors, buses, mobile homes, campers, gliders,airplanes, and tanks.

The method 80 comprises obtaining a coil 82 of a metal material havingan alloy composition according to the present technology and cutting ablank 84 from the coil 82. The method also comprises austenitizing theblank by heating the blank in a furnace 86 to a temperature above itsAc3 temperature to form a heated blank comprising austenite. Optionallyby a robotic arm 88, the heated blank is transferred to a press 90.Here, the method 80 comprises stamping the heated blank into apredetermined shape to form a stamped object, and quenching the stampedobject to form a shaped steel object 92, wherein the shaped steel object92 comprises martensite. The method 80 is free of a pre-oxidation step,of a coating step, and of a descaling step (e.g., shot blasting).

In one embodiment, the quenching is performed traditionally by coolingthe shaped object at a rate described above until the stamped objectreaches a temperature below an Mf temperature of the alloy composition.Here, the shaped steel object has a microstructure that is fullymartensite or substantially fully martensite. By “substantially fully”it is meant that greater than or equal to about 80%, greater than orequal to about 85%, greater than or equal to about 90%, or greater thanor equal to about 95% of the microstructure is martensite.

In another embodiment, the quenching comprises a quench and partitioningprocess as described above. Here, the method comprises decreasing thetemperature of the stamped object until the stamped object has atemperature between an Ms temperature of the alloy composition and a Mftemperature of the alloy composition, incubating the stamped object at apartitioning temperature wherein carbon (C) is partitioned frommartensite into austenite, and then decreasing austenite's Mftemperature below room temperature. The partitioning temperature can bethe temperature between the Ms and Mf temperatures at which the coolingis stopped, a temperature higher than the temperature between the Ms andMf temperatures at which the cooling is stopped, or a temperature lowerthan the temperature between the Ms and Mf temperatures at which thecooling is stopped. Partitioning is performed at the partitioningtemperature for a time of greater than or equal to about 0.01 min toless than or equal to about 20 min. After the quench and partitioningprocess, the shaped steel object has a microstructure comprisingmartensite and RA. Due to the high Si content of the alloy composition,the microstructure of the shaped steel object is substantially free ofcementite. As used herein, “substantially free” refers to less than orequal to about 10%, less than or equal to about 5%, or less than orequal to about 1%.

In one variation of the method 80, the coil 82 comprises AHSS for a coldstamping. Here, as shown by the dotted line, after the blank 84 is cutfrom the coil 82, it is transferred to the press 90, optionally by wayof the robotic arm 88. The method 80 comprises stamping the blank 84into a predetermined shape at ambient temperature to form the shapedsteel object 92. Although the shaped steel object can be bare, invarious embodiments, the method also includes disposing a zinc (Zn)coating on the shaped steel object.

The foregoing description of the embodiments has been provided forpurposes of illustration and description. It is not intended to beexhaustive or to limit the disclosure. Individual elements or featuresof a particular embodiment are generally not limited to that particularembodiment, but, where applicable, are interchangeable and can be usedin a selected embodiment, even if not specifically shown or described.The same may also be varied in many ways. Such variations are not to beregarded as a departure from the disclosure, and all such modificationsare intended to be included within the scope of the disclosure.

What is claimed is:
 1. An alloy composition comprising: chromium (Cr) ata concentration of greater than or equal to about 0.5 wt. % to less thanor equal to about 9 wt. %; carbon (C) at a concentration of greater thanor equal to about 0.15 wt. % to less than or equal to about 0.5 wt. %;manganese (Mn) at a concentration of greater than or equal to about 0wt. % to less than or equal to about 3 wt. %; silicon (Si) at aconcentration of greater than or equal to about 0.5 wt. % to less thanor equal to about 2 wt. %; and a balance of the alloy composition beingiron.
 2. The alloy composition according to claim 1, wherein the alloycomposition comprises Si at a concentration of greater than or equal toabout 0.6 wt. % to less than or equal to about 1.5 wt. %.
 3. The alloycomposition according to claim 1, wherein the alloy compositioncomprises Cr at a concentration of greater than or equal to about 2 wt.% to less than or equal to about 3 wt. %.
 4. The alloy compositionaccording to claim 1, wherein the alloy composition further comprises:aluminum (Al) at a concentration of greater than or equal to about 0 wt.% to less than or equal to about 5 wt. %.
 5. The alloy compositionaccording to claim 1, wherein the alloy composition further comprises:nitrogen (N) at a concentration of greater than or equal to about 0 wt.% to less than or equal to about 0.01 wt. %.
 6. The alloy compositionaccording to claim 1, wherein the alloy composition further comprises atleast one of: molybdenum (Mo) at a concentration of greater than orequal to about 0 wt. % to less than or equal to about 1 wt. %; nickel(Ni) at a concentration of greater than or equal to about 0 wt. % toless than or equal to about 1 wt. %; boron (B) at a concentration ofgreater than or equal to about 0 wt. % to less than or equal to about0.01 wt. %; niobium (Nb) at a concentration of greater than or equal toabout 0 wt. % to less than or equal to about 0.5 wt. %; and vanadium (V)at a concentration of greater than or equal to about 0 wt. % to lessthan or equal to about 0.5 wt. %.
 7. The alloy composition according toclaim 1, wherein the alloy composition is in the form of an alloy coil.8. The alloy composition according to claim 7, wherein the alloy coilcomprises ferrite, martensite and retained austenite (RA).
 9. The alloycomposition according to claim 7, wherein the alloy composition has beensubjected to a quench and partitioning process.
 10. A hot stampingmethod of forming a shaped steel object, the hot stamping methodcomprising: austenitizing a blank comprising an alloy compositionaccording to claim 1; stamping the austenitized blank to form a shapedobject; and quenching the shaped object to form the shaped steel object.11. A cold stamping method of forming a shaped steel object, the coldstamping method comprising: cutting a blank from a coil comprising analloy composition according to claim 1, wherein the alloy compositionhas been subjected to a quench and partitioning process; and stampingthe blank into a predetermined shape at ambient temperature to form theshaped steel object.
 12. A method of forming a shaped steel object; themethod comprising: cutting a blank from a coil of an alloy compositioncomprising: chromium (Cr) at a concentration of greater than or equal toabout 0.5 wt. % to less than or equal to about 9 wt. %, carbon (C) at aconcentration of greater than or equal to about 0.15 wt. % to less thanor equal to about 0.5 wt. %, manganese (Mn) at a concentration ofgreater than or equal to about 0 wt. % to less than or equal to about 3wt. %, silicon (Si) at a concentration of greater than or equal to about0.5 wt. % to less than or equal to about 2 wt. %, and a balance of thealloy composition being iron; heating the blank to a temperature abovean upper critical temperature (Ac3) of the alloy composition to form aheated blank comprising austenite; stamping the heated blank into apredetermined shape to form a stamped object; and quenching the stampedobject to form the shaped steel object, wherein the shaped steel objectcomprises martensite.
 13. The method according to claim 12, wherein thequenching comprises decreasing the temperature of the stamped object ata rate of greater than or equal to about 15° C./s until the stampedobject reaches a temperature below a martensite finish (Mf) temperatureof the alloy composition.
 14. The method according to claim 12, whereinthe method is free from pre-oxidizing the alloy composition, coating theshaped steel object, and shot blasting.
 15. The method according toclaim 12, wherein the quenching comprises a quench and partitioningprocess, wherein the quench and partitioning process comprises:decreasing the temperature of the stamped object until the stampedobject has a temperature between a martensite start (Ms) temperature ofthe alloy composition and a martensite finish (Mf) temperature of thealloy composition; incubating the stamped object at a partitioningtemperature wherein carbon (C) is partitioned from martensite intoaustenite; and decreasing an austenite Mf temperature below roomtemperature.
 16. The method according to claim 15, wherein the quenchand partitioning process forms the shaped steel object, wherein theshaped steel object comprises ferrite, martensite and retained austenite(RA).
 17. The method according to claim 16, wherein the shaped steelobject is substantially free of cementite.
 18. A method of forming ashaped steel object; the method comprising: cutting a blank from a coilof an advanced high strength steel (AHSS); and stamping the blank into apredetermined shape at ambient temperature to form the shaped steelobject, wherein the AHSS is made by subjecting an alloy composition to aquench and partitioning process, the alloy composition comprising:chromium (Cr) at a concentration of greater than or equal to about 0.5wt. % to less than or equal to about 9 wt. %, carbon (C) at aconcentration of greater than or equal to about 0.15 wt. % to less thanor equal to about 0.5 wt. %, manganese (Mn) at a concentration ofgreater than or equal to about 0 wt. % to less than or equal to about 3wt. %, silicon (Si) at a concentration of greater than or equal to about0.5 wt. % to less than or equal to about 2 wt. %, and a balance of thealloy composition being iron.
 19. The method according to claim 18,wherein the AHSS is substantially free of an oxide layer.
 20. The methodaccording to claim 18, wherein the shaped steel object is bare or zinc(Zn coated).